1. Field of the Invention
The present invention relates to an actuator, and in particular relates to an actuator of the type employing a two-degree-of-freedom vibration system.
2. Description of the Prior Art
There is known a polygon mirror (rotary polyhedron) as an actuator provided in laser printers, for example. In such a printer, in order to achieve higher-resolution and higher-quality printed output as well as higher-speed printing, it is necessary to rotate the polygon mirror at higher speed. Currently, an air bearing is used to rotate the polygon mirror at high speed with stability. However, there is a problem in that it is difficult to rotate the polygon mirror at much higher speed than the speed available at the present. Further, although a larger motor is required in order to rotate the polygon mirror at higher speed, use of such a larger motor gives rise to a problem in that it is difficult to miniaturize the size of an apparatus in which the polygon mirror is used. Furthermore, use of such a polygon mirror gives rise to another problem in that the structure of the apparatus becomes necessarily complicated, thus leading to increased manufacturing cost.
On the other hand, a single-degree-of-freedom torsional vibrator as shown in FIG. 8 has been proposed since the early stages of research in the field of actuators. Since this vibrator uses flat electrodes which are arranged in parallel with each other, it can have quite simple structure (see K. E. Petersen: “Silicon Torsional Scanning mirror”, IBMJ. Res. Develop., Vol. 24 (1980), P. 631, for example). Further, a single-degree-of-freedom electrostatic drive type vibrator obtained by modifying the torsional vibrator described above so as to have a cantilever structure has also been proposed (see Kawamura et al. “Research in micromechanics using Si”, Proceedings of the Japan Society for Precision Engineering Autumn Conference (1986), P. 753, for example).
FIG. 8 shows such a single-degree-of-freedom electrostatic drive type torsional vibrator. In the torsional vibrator shown in FIG. 8, a movable electrode plate 1300 made of monocrystalline silicon is fixed at end fixing portions 1300a thereof to the both ends of a glass substrate 1000 through spacers 1200. The movable electrode plate 1300 includes a movable electrode portion 1300c which is supported by the end fixing portions 1300a through narrow torsion bars 1300b. Further, a fixed electrode 1400 is provided on the glass substrate 1000 so as to be opposed to the movable electrode portion 1300c through a predetermined electrode interval. Specifically, the fixed electrode 1400 is arranged in parallel with the movable electrode portion 1300c through the electrode interval therebetween. The fixed electrode 1400 is connected to the movable electrode plate 1300 via a switch 1600 and a power source 1500.
In the torsional vibrator having the structure described above, when a voltage is applied across the movable electrode portion 1300c and the fixed electrode 1400, the movable electrode portion 1300c rotates around the axis of the torsion bars 1300b due to electrostatic attraction. Since electrostatic attraction is inversely proportional to the square of an electrode interval, it is preferable for this type of electrostatic actuator to have a small electrode interval between the movable electrode portion 1300c and the fixed electrode 1400. However, in such a single-degree-of-freedom torsional vibrator described above, the movable electrode portion 1300c which serves as a movable portion is also provided with the electrode. Therefore, if the electrode interval becomes too small, a movable range (rotational angle) of the movable electrode portion 1300c is necessarily limited. On the other hand, in order to enlarge the movable range of the movable electrode portion 1300c, it is necessary to widen the electrode interval and this in turn needs a large driving voltage. Namely, such a single-degree-of-freedom torsional vibrator described above involves a problem in that it is difficult to achieve both of low-voltage driving and large displacement.